Aaron Carroll:
    Last week I talked to you about dietary cholesterol and how the existing randomized control trials warned us that restricting our intake wouldn't work. Now it appears those guidelines might be changed -- decades later. Cholesterol isn't the only recommendation that's controversial though; so are the ones on fat. Prepared to get annoyed. This is Healthcare Triage.

    (Healthcare Triage Intro)

    For decades, consuming a low percentage of your daily calories from fat has been the mainstay of many, many dietary recommendations. That's because it's been widely thought that doing so would reduce your chances of having coronary heart disease. The US Select Committee on Nutrition and Human Needs announced in 1977 and the UK National Advisory Committee on Nutritional Education announced in 1983 dietary recommendations to reduce fat intake. Specifically they told everyone to reduce overall fat consumption to 30% of total energy intake and to reduce saturated fat consumption to 10%.
    Most of the evidence for that recommendation came from epidemiologic studies, which can be flawed. Remember, they can be great at correlation, not always so much on causation.
    And this settling for epidemiologic data happens far more often than we would like. Sometimes these kinds of studies are all we can get because you can't do a randomized control trial to prove causality. We'll never have a randomized control trial for smoking and cancer, for instance, because the evidence from cohort and case control studies is so compelling that telling a random population to smoke to see if it's harmful would be unethical. But there is no reason we couldn't randomize people to diets.
    Turns out we have. In fact, randomized control trials existed when the low-fat diet guidelines were published, and it appears they were ignored. Just a few weeks ago a study was published in the journal Open Heart where researchers performed a systematic review and meta-analysis of the existing randomized control trials available when the guideline was announced. They wanted to explore what might have been considered by those creating the guideline back when they did so.
    Before 1983, six randomized control trials involving more than 2400 men had been conducted. None were explicate studies of the recommended diet, but all explored the relationship between dietary fat, cholesterol, and mortality. Five of them were secondary prevention trials, meaning that they only involved men with known problems already. Only one included healthy participants, who would be at lower risk and therefore would likely have even less benefit from dietary changes.
    That's a lot of participants. Moreover, many of them were at high risk, and in all of them there was no significant difference in the rate of death from coronary heart disease between them. There were also no differences in all caused mortality, which is the metric that matters. 
    And if you can't find a difference in people at really high risk then the chance of finding a difference doing this in the general population is even lower. Now, some of the studies did show that cholesterol levels went down more in the groups that ate low-fat diets. But these groups didn't have different clinical outcomes, and that's what we really care about.
    Still, many will point to the fact that many of these recommendations might have been responsible for a lowering of cholesterol and justify a low-fat diet that way, but the difference between them was small. Mean cholesterol went down 13% in the intervention groups, but it went down 7% in the control groups. Overall, the difference was only 6%.
    And it's important to remember that even if the cholesterol went down, the clinical outcomes didn't change. No difference in death from coronary heart disease, no difference in all causes of mortality, and that's why we're doing the recommendations.
    I'm pretty immersed in the medical literature and all of this is still shocking to me. It's hard to overestimate the impact of the dietary guidelines. Hundreds of millions of people changed their diets based on these recommendations. They consumed less fat, they avoided cholesterol, and they reduced their intake of salt, and you've already seen our video on how that one's gotten off the rails. And since pretty much a lot of calories come from fat, protein, or carbohydrates reducing your consumption of one means that you have to increase your consumption of another. As meat, especially red meat, had also been shunned by guidelines, this meant that many people began to increase their consumption of carbohydrates.
    Decades later, it's not hard to find evidence this might have been a bad move. Many now believe that excessive carbohydrate consumption many contributing to the obesity and diabetes epidemics. A Cochrane Review of all randomized controlled trials of reduced- or modified-dietary-fat interventions found that replacing fat with carbohydrates is not protective even against cardiovascular events, let alone death.
    Now there still exists some evidence that the type of fats people are consuming might have some link to health. Reducing your consumption of saturated fat could reduce the number of cardiovascular events a person might have. It still doesn't conclusively reduce mortality or deaths but at least it appears to do something.
    But the authors of the Cochrane Review explicitly said, and I quote, "Dietary change to reduce saturated fat and partially replace it with unsaturated fats appears to reduce the incidence of cardiovascular events, but replacing the saturated fat with carbohydrate (creating a low-fat diet) was not clearly protective of cardiovascular events (despite small improvements in weight, body mass index, and total and LDL cholesterol). The protective effect was seen almost exclusively in those who continue to modify their diet over at least two years, and in studies of men (not those of women)."
    Is that what recommendations say? I don't think so. It's frustrating enough when we over-read the results of epidemiologic studies and make the mistake of believe that correlation is the same as causation. It's maddening, however, when we ignore the results of randomized control trials which are capable of proving causation, to continue down the wrong path.
    In reviewing the literature, it's hard to come away with the sense that anyone knows for sure what diet should be recommended to all Americans. I understand people's frustration with the continuing shifting sands in nutrition recommendations. For decades they've been told what to eat because science says so. Unfortunately, that doesn't appear to be true. That's tragic not only because reduces people's faith in science as a whole but also because it may have cost some people better or potentially even their lives.